1. Field of the Invention
The present invention relates to a motor driving device for driving a three-phase motor.
2. Description of the Related Art
In related arts, as a three-phase motor, a three-phase brushless motor is known. The three-phase brushless motor does not involve the abrasion of a brush to provide good durability, resulting in the three-phase brushless motor being widely used accordingly. In the widely used type of a three-phase brushless motor, rotation torque is generated by three phases of alternating current flowing through three-phase coils. In order to control the three phases of current, a rectangular wave has been widely used due to the easiness in configuring a driving circuit. However, in recent years, sinusoidal wave current-based driving control with the use of a sinusoidal wave is being performed to improve quietness and driving efficiency. Further, a vector control scheme increasingly becomes a usual practice in which amplitudes and phases of the three-phase sinusoidal wave currents are controlled with high precision. This scheme requires detection of phase currents corresponding to at least two phases. However, using two sensors in detecting two phase currents increases a cost. Accordingly, an alternative scheme is proposed in which a bus current between an inverter and a DC power supply is detected by using one current sensor so as to reproduce an original phase current (at least two phases of the current) (for example, see Japanese Patent Application Laid-open No. 2008-99542). This scheme is called a one-shunt current detection scheme (single-shunt current detection scheme). In the one-shunt current detection scheme, an output signal of a current sensor is sampled at appropriate timing to detect two phases of current: a phase (the maximum phase) of current providing the maximum voltage level; and a phase (the minimum phase) of current providing the minimum voltage level.
The above-described one-shunt current detection scheme suppresses the cost for the sensor. However, the bus current appears only when one or two phases in a three-phase pulse width modulation (PWM) inverter is in the ON state. Therefore, in order to obtain current levels of two phases from the bus current, it is necessary to perform sampling on two bus current levels: the level of a bus current during an interval in which only one phase is ON and the level of a bus current during an interval in which two phases are ON.
However, because three-phase PWM pulses are generated by a high speed modulation of a three-phase sinusoidal wave current, there is certainly an occasion in which two pulse widths become very close or equal to each other and, in its vicinity, the interval for sampling the bus current becomes very short. In general, it is difficult to perform sampling on the bus current during an interval in which two PWM pulse widths are close to each other partly because a detection result of the bus current may become obscure due to the bandwidth limiting in a current sensor or a detection amplifier and partly because there is a limitation on the conversion speed of an AC/DC converter (ADC) generally used as a sampling unit. In other words, although two phase currents can be detected in a case in which the respective voltage levels thereof are sufficiently far from each other, two phase currents cannot be detected when the maximum phase voltage is close to the intermediate phase voltage or when the minimum phase voltage is close to the intermediate phase voltage.
Accordingly, in Japanese Patent Application Laid-open No. 2008-99542 and the literature disclosed as a reference in the section of the related art, during the interval in which two PWM pulse widths are close to each other, the pulse widths are corrected not to be close to each other. For example, Japanese Patent Application Laid-open No. 2008-99542 discloses a motor control device which performs a vector control on a three-phase motor based on the one-shunt current detection scheme. In this motor control device, an ab coordinate system is defined such that the ab coordinate system rotates by an electric angle of 60 degrees in a stepwise manner according to a phase of a voltage command vector as viewed from the U phase axis. Further, in a rotating coordinate system which rotates at the same speed as a magnetic flux exerted by a permanent magnet which is provided in a rotor of the motor, given that the direction of the magnetic flux exerted by the permanent magnet is set to be the d axis and the phase rotated by an electric angle of 90 degrees from the d axis is set to be the q axis, a coordinate, employing these axes as the axes of the coordinates is defined as a dq coordinate. Then, the dq coordinates of the voltage command vector are converted into ab coordinates, and it is determined whether or not two phase currents are detectable based on the magnitudes of the coordinate elements (va, vb) of the voltage command vector in the ab coordinate system without correcting the voltage command vector. In a case in which the correction is necessary, the magnitudes of the respective coordinate elements are corrected, and three-phase voltage command values to be supplied to the inverters are created from the corrected voltage command vector.
However, in the conventional pulse width correction scheme described above, the algorithm is likely to be complicated, which makes the scheme difficult to be implemented using an inexpensive circuit or microprocessor. The timing at which twice of sampling on the bus current are performed can be at any position of the PWM interval, before and/or after the correction. In other words, the sampling timing is not concentrated in a given section (the first half section or the second half section) of the PWM interval. Therefore, an ADC serving as a sampling unit operates all the time, preventing the ADC to be used for other purposes or to be used for current detection of other motors by means of time division.